The present invention relates to a method for the detection of a block raster in image processing.
In image processing it is known that images may be compressed prior to data transmission or storage to reduce the transmission rate or the memory capacity required to store the image. There are methods in which this type of image compression is done in blocks. In these methods, an image to be compressed is divided into a plurality of image blocks, for example image blocks composed of 8×8 pixels, and the image data of the blocks obtained in this manner are compressed all at the same time.
As a result of this block-by-block image processing of an image to be compressed, after a decompression of the image, block-like structures may be present in the decompressed image which are called block artifacts or blocking artifacts. Blocking artifacts of this type are visible edges along the original block boundaries. Blocking artifacts of this type may be detected by appropriate algorithms and may then be removed or at least reduced by appropriate filtering. A method of this type for the reduction of blocking artifacts logically includes a method for the detection of the original block raster. It may thereby be ensured that filtering for the reduction of blocking artifacts takes place only along original block boundaries. This approach eliminates or at least reduces the risk that edges that are part of the image content are erroneously detected as blocking artifacts and are thereby filtered out.
If the block raster is known, i.e., if the position of the block raster within the image and the distances between the individual block boundaries are known, no further effort is necessary. This information about the block raster may then be used directly to search for blocking artifacts only along the block boundaries and to reduce them if necessary.
A more difficult situation arises when an image is scaled after decompression even before additional image processing measures, such as, for example, processes to reduce blocking artifacts, are applied. As a result of an image scaling of this type, visible edges may be “distributed” along the original raster boundaries over a plurality of pixel boundaries. A sudden signal step change at the original raster boundary then results in multiple small signal step changes at neighboring pixel boundaries, which makes it difficult to detect the original block raster.
An object of the present invention is to reliably detect a raster which results, for example, from a block-by-block compression of an image, and specifically when the image has been scaled after the performance of block-by-block processing.